U.S. patent number 4,822,459 [Application Number 07/091,148] was granted by the patent office on 1989-04-18 for lead oxide-coated electrode for use in electrolysis and process for producing the same.
This patent grant is currently assigned to Agency of Industrial Science and Technology (MITI), Permelec Lectrode Ltd.. Invention is credited to Takayuki Shimamune, Minoru Ueda, Akio Watanabe.
United States Patent |
4,822,459 |
Ueda , et al. |
April 18, 1989 |
Lead oxide-coated electrode for use in electrolysis and process for
producing the same
Abstract
A lead oxide-coated electrode for use in electrolysis is
disclosed, which comprises a primary layer comprising platinum
and/or platinum oxide, an intermediate layer comprising
.alpha.-PbO.sub.2, and a coating layer comprising .beta.-PbO.sub.2
successively coated on a substrate comprising a corrosion resistant
metal. The electrode has a strong and durable lead oxide coating
firmly bonded to the substrate, does not suffer from passivation,
resistance increase, etc. and can be used stably for a long time at
high current density. A process for producing the electrode is also
disclosed.
Inventors: |
Ueda; Minoru (Ibaraki,
JP), Watanabe; Akio (Ibaraki, JP),
Shimamune; Takayuki (Tokyo, JP) |
Assignee: |
Agency of Industrial Science and
Technology (MITI) (Tokyo, JP)
Permelec Lectrode Ltd. (Kanagawa, JP)
|
Family
ID: |
26512706 |
Appl.
No.: |
07/091,148 |
Filed: |
August 31, 1987 |
Foreign Application Priority Data
|
|
|
|
|
Aug 29, 1986 [JP] |
|
|
61-201287 |
Aug 29, 1986 [JP] |
|
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61-201288 |
|
Current U.S.
Class: |
205/109;
427/126.3; 427/226; 427/419.3; 428/421; 428/701; 427/126.5;
427/327; 428/469; 204/290.08; 204/290.06 |
Current CPC
Class: |
C25B
11/093 (20210101); C25B 11/054 (20210101); C25B
11/04 (20130101); Y10T 428/3154 (20150401) |
Current International
Class: |
C25B
11/16 (20060101); C25B 11/04 (20060101); C25B
11/00 (20060101); C25D 005/00 () |
Field of
Search: |
;204/29F,38.1,56.1,40
;427/126.1,126.3,125.5,226,372.2,419.2,419.3,327,299,190
;428/421,469,701 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Chemical Abstracts, vol. 89, No. 22, Nov. 1978, p. 510, Abstract
No. 187959x. .
Chemical Abstracts, vol. 101, No. 4, Jul. 1984, p. 424, Abstract
No. 30198s..
|
Primary Examiner: Niebling; John F.
Assistant Examiner: Gorgos; Kathryn
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
What is claimed is:
1. A lead oxide-coated electrode for use in electrolysis which
comprises (1) a primary layer comprising platinum, palladium oxide,
or mixtures thereof, (2) an intermediate layer comprising
.alpha.-PbO.sub.2, and (3) a coating layer comprising
.beta.-PbO.sub.2 and having dispersed therein a corrosion resistant
and electrochemically inactive granular material, fibrous material
or mixtures thereof, each of (1), (2) and (3) successively coated
on a substrate comprising a corrosion resistant metal.
2. An electrode as claimed in claim 1, wherein the corrosion
resistant metal of the substrate is titanium, zirconium, niobium,
tantalum or a base alloy thereof.
3. An electrode as claimed in claim 1, wherein the primary layer
(1) comprises platinum, palladium oxide or mixtures thereof and at
least one of oxides of titanium, tantalum or tin.
4. An electrode as claimed in claim 1, wherein the thickness of the
intermediate layer (2) is from 20 to 500 .mu.m.
5. An electrode as claimed in claim 1, wherein the granular
material, fibrous material or mixtures thereof is a metal selected
from group IV and group V elements of the periodic table or oxides,
carbides, nitrides or borides of said metals.
6. An electrode as claimed in claim 1, wherein the granular
material, fibrous material or mixtures thereof comprises a fluoro
resin.
7. A process for producing a lead oxide-coated electrode for use in
electrolysis, which comprises successively forming, on a substrate
comprising a corrosion resistant metal, (1) a primary layer
comprising platinum, palladium oxide, or mixtures thereof, (2) an
intermediate layer comprising .alpha.-PbO.sub.2, and (3) a coating
layer comprising .beta.-PbO.sub.2 electrolytically formed from an
acidic bath containing lead ions and having dispersed therein a
corrosion resistant and electrochemically inactive granular
material, fibrous material or mixtures thereof.
8. A process as claimed in claim 7, wherein the surface of the
corrosion resistant metal substrate is subjected to blasting,
pickling or a combination of blasting and pickling prior to forming
the primary layer.
9. A process as claimed in claim 7, wherein the primary layer (1)
is formed by coating a solution containing a heat decomposable salt
of platinum, palladium or mixtures thereof on the substrate and
then subjecting the coated solution to heat treatment.
10. A process as claimed in claim 7, wherein the intermediate layer
(2) is electrolytically formed from an alkaline bath containing
lead ions.
11. A process for producing a lead oxide-coated electrode for use
in electrolysis, which comprises successively forming, on a
substrate comprising a corrosion resistant metal, (1) a primary
layer comprising platinum, palladium oxide or mixtures thereof, (2)
an intermediate layer comprising .alpha.-PbO.sub.2, and (3) a
coating layer comprising .beta.-PbO.sub.2, electrically formed from
an acidic bath containing lead ions having dispersed therein a
corrosion resistant and electrochemically inactive granular
material, fibrous material or mixtures thereof and wherein the
coated layer (3) is formed by repeating the step of forming said
.beta.-PbO.sub.2 layer with said corrosion resistant and
electrochemically inactive granular, fibrous material or mixtures
thereof.
12. A process as claimed in claim 11, wherein the surface of the
corrosion resistant metal substrate is subjected to blasting,
pickling or a combination of blasting and pickling prior to forming
the primary layer (1).
13. A process as claimed in claim 12, wherein the primary layer (1)
is formed by coating a solution containing a heat decomposable salt
of platinum, palladium or mixtures thereof on the substrate and
then subjecting the coated solution to heat treatment.
14. A process as claimed in claim 11, wherein the intermediate
layer (2) is electrolytically formed from an alkaline bath
containing lead ions.
Description
FIELD OF THE INVENTION
This invention relates to a lead oxide-coated electrode for use in
electrolysis and, more specifically, it relates to a lead
oxide-coated electrode for use in electrolysis suitable as an anode
for generating oxygen or ozone, for anodic oxidation, etc. in the
electrolysis of an aqueous acidic solution or organic-containing
solution, etc. This invention also relates to a process for
producing such an electrode.
BACKGROUND OF THE INVENTION
A metal electrode coated with lead oxide has been known to be
suitable as an electrode for use in electrolysis requiring
corrosion resistance or high oxygen overvoltage, for instance,
electrolysis for the generation of oxygen, anodic oxidation,
electroplating, electrolysis of organic materials, electrolytic
treatment of waste water, etc., and various improvements have been
made in the electrode. However, since practical problems have still
been present, these electrodes have not yet been used generally for
industrial applications.
Lead oxide used as the electrode includes two types, that is,
rhombic .alpha.-PbO.sub.2 and tetragonal .beta.-PbO.sub.2 of a
rutile type structure. While .alpha.-PbO.sub.2 shows poor corrosion
reistance when used as an anode for electrolysis as compared with
.beta.-PbO.sub.2, .alpha.-PbO.sub.2 with no substantial internal
strain can be obtained by electrodepositions when it is
electrolytically formed on a metal substrate such as titanium. On
the other hand, while .beta.-PbO.sub.2 has good electroconductivity
and good corrosion resistance, if .beta.-PbO.sub.2 is
electrolytically formed, internal straining due to
electrodeposition is generally increased to cause cracking or
deteriorate the bondability with the metal substrate.
In addition, these PbO.sub.2 layers are generally poor in
mechanical strength, lack processability and passivate the metal
substrate, such as titanium, due to the oxidizing effect of
PbO.sub.2 thereby making electroconduction difficult.
Among the problems as described above, for improving the
bondability between the metal substrate and lead oxide, it has been
known to adopt a countermeasure for increasing the surface area of
the metal substrate, as described, for example, in Japanese Patent
Publication Nos. 31396/83 and 34235/84.
Further, there has also been proposed a method of partially
depositing a platinum group metal on a metal substrate by electric
discharge as described in Japanese Patent Publication No. 45835/82,
and a method of disposing fine noble metal portion areas in a
distributed manner on the surface of the substrate as described in
Japanese Patent Publication No. 32435/79, for preventing the
passivation of the metal substrate. According to these methods,
however, a large amount of expensive noble metal is needed, which
is not practical and, in addition, they involve complicated
procedures.
There have also been many proposals relating to coating a lead
oxide layer on a metal substrate by way of various primary layers
or intermediate layers. For example, there is a method of
previously coating a titanium (IV) compound on the surface of a
titanium substrate as described in Japanese Patent Publication No.
45191/78, a method of disposing a thin flash layer of a platinum
group metal as described in Japanese Patent Publication No.
9236/81, a method of disposing an intermediate layer made of a
platinum group metal or metal oxide as described in Japanese Patent
Publication Nos. 30957/83, 31396/83, and 34235/84, a method of
disposing an intermediate layer of a carbide and boride of a group
IV-VI element and/or silicide of a sub-group of group IV - VI
elements and/or silicon carbide as described in Japanese Patent
Publication No. 72878/75, and a method of disposing a semiconductor
intermediate layer made of a tin compound and an antimony compound
as described in Japanese Patent Application (OPI) No. 82680/77 (the
term "OPI" as used herein refers to "unexamined published patent
application).
Among these methods, the method of disposing the intermediate layer
containing the platinum group metal or the oxide thereof is not
practical since the intermediate layer itself is extremely
expensive. In addition, some of these materials are usually
employed as an electrode active substance and, since they show a
low oxygen overvoltage as an anode as compared with lead oxide, if
electrolytes intrude through pin holes, etc. in the lead oxide
coating layer, the intermediate layer acts as an anode to evolve
gases due to the electrolytic action at the surface of the
intermediate layer to possibly result in peeling and destruction of
the lead oxide layer. Further, in the method of disposing an
intermediate layer not containing a platinum metal group such as an
intermediate layer of a semiconductor material of tin and antimony
compounds, although there is less possibility that the intermediate
layer will act as an anode, the electroconductivity is insufficient
leaving a problem for electric current supply. Further, since the
radius of lead ions is 0.78 .ANG. for Pb.sup.4+ (6-coordination),
which is greater as compared with 0.69 .ANG. for Sn.sup.4+ or 0.61
.ANG. for Ti.sup.4+, it is difficult to firmly bond the
intermediate layer and the lead oxide layer to each other by fusion
or by forming a solid-solution. Further, since the .beta.-PbO.sub.2
layer has a great ion radius as described above, considerable
stresses occur within it, it being the rutile type oxide, and
complete bonding is difficult even to the intermediate layer.
In view of the above, use of .alpha.-PbO.sub.2 with less strain has
been proposed and alternate layers of .alpha.-PbO.sub.2 and
.beta.-PbO.sub.2 are disclosed in Japanese Patent Publication No.
9472/80. It is also known to apply silver plating to the surface of
a metal substrate and dispose .alpha.-PbO.sub.2 further thereover
as described in Japanese Patent Publication No. 23947/76. While
these methods can provide a lead oxide layer with less strain,
there have still been problems such as poor corrosion resistance of
.alpha.-PbO.sub.2, solution of silver in an acidic solution, etc.,
and they can not yet be said to be satisfactory.
As has been described above, known lead oxide-coated electrodes
involve various problems in view of their performance and
manufacture and no practically excellent electrode had been
obtained yet.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a lead
oxide-coated electrode having a long life time and satisfactory
stability, and having .beta.-PbO.sub.2 coatings formed on a metal
substrate which is dense and excellent in the bondability and shows
less internal strain due to electrodepositions.
This invention firstly provides a lead oxide-coated electrode for
use in electrolysis which comprises a primary layer comprising
platinum and/or palladium oxide, an intermediate layer comprising
.alpha.-PbO.sub.2, and a coating layer comprising .beta.-PbO.sub.2
successively coated on a substrate comprising a corrosion resistant
metal.
The present invention also provides a process for producing the
above-described lead oxide-coated electrode for use in electrolysis
which comprises successively forming on a substrate comprising a
corrosion resistant metal, a primary layer comprising platinum
and/or palladium oxide, an intermediate layer comprising
.alpha.-PbO.sub.2, and a coating layer comprising
.beta.-PbO.sub.2.
DETAILED DESCRIPTION OF THE INVENTION
This invention will now be described in more detail.
In the present invention, a corrosion-resistant metal is used as
the substrate for the electrode, and titanium, zirconium, niobium
and tantalum which are collectively referred to as valve metals or
a basic alloy thereof are preferred. The metal substrate has no
particular restriction on the shape, and may be a plate, apertured
plate, rod-like member, expanded metal, mesh-like member, etc.
Further, since a relatively thick layer of lead oxide is
subsequently coated on the substrate, it is preferred to apply a
roughing treatment to the surface thereby increasing the deposition
area. As the roughing treatment, a blasting treatment can be used.
The blasting treatment is usually performed using grits or sand
having a relatively large particle size. Also, it is desirable to
form a fine unevenness on the surface of the substrate by way of
pickling using oxalic acid, sulfuric acid, hydrochloric acid, etc.
for improving the adherence with the primary layer, as well as to
clean or activate the surface.
A primary layer containing platinum and/or palladium oxide is
formed on the surface of the thus prepared metal substrate for
protecting the substrate and improving the bondability with the
intermediate layer. While platinum is usually used in the form of a
metal, it is necessary that palladium is used in the form of its
oxide since the corrosion resistance of palladium in the form of a
metal is poor.
For forming such a primary layer, heat decomposing is usually
preferred and a primary coating containing platinum and/or
palladium oxide can be obtained with ease by coating a solution
containing a heat decomposable salt of platinum and/or palladium,
drying and then heating and performing a heat decomposing treatment
in air.
The platinum and/or palladium oxides are used for the primary layer
in the present invention, because these materials have a
sufficiently high oxygen generating over-voltage. Specifically, the
lead oxide electrode is often used as an anode in an aqueous
solution and the reaction mainly comprises evolution of oxygen. On
this case, it has been found that since the lead oxide has a high
oxygen generation overvoltage, it is necessary to increase the
overvoltage of the primary layer. The above-mentioned materials
have been found to sufficiently satisfy this requirement.
Electroconductive oxides not using a noble metal, for example, tin
oxide or titanium oxide have a sufficiently high overvoltage but
have poor electroconductivity, whereas noble metals other than
platinum and palladium, such as ruthenium, iridium and rhodium have
good electroconductivity but have a lower oxygen overvoltage than
that of lead oxide, and thus they are not suitable for use in the
present invention.
Although a sufficient effect can be attained by using only platinum
and/or palladium oxide in the primary layer, the platinum and/or
palladium oxide may be used in admixture with other metal oxides
for improving the bondability with the substrate and reducing the
amount of expensive noble metal used. As such metal oxides,
titanium oxide, tantalum oxide-doped titanium oxide, tin oxide,
etc. can be used, by which additional effects can also be expected
such as improvement in the corrosion resistance of the primary
layer itself and an increase in the oxygen overvoltage. The amount
of the other metal oxide in the composition is preferably from 0 to
90 mol% based on the total amount of the primary layer.
A suitable coating thickness of the primary layer is from about
0.05 to 3 .mu.m. A sufficient coating for the substrate can not be
obtained if the thickness of the primary layer is less than 0.05
.mu.m, whereas electrical resistance tends to be increased if the
thickness of the primary layer exceeds 3 .mu.m.
The heat decomposing conditions for forming the primary layer are
properly selected depending on the composition of the coating, and
heat treatment may usually be applied in an oxidative atmosphere
such as air at a temperature of from 300.degree. to 700.degree. C.
for 5 to 30 minutes. The desired coating thickness can be obtained
by repeating the coating and heating procedures for the coating
solution. For coating the primary layer, coatings of different
compositions may be applied in an adequate order thereby obtaining,
as a whole, a primary layer of a desired composition, as well as
repeating the coating of an identical composition. In the case of
applying repeated coatings of different compositions, since the
thickness of each coating is thin, ingredients are diffused between
layers upon heat treatment of each layer thereby enabling a primary
layer coating of sufficiently high electroconductivity as a whole
to be obtained.
After forming the primary layer, an intermediate layer comprising
.alpha.-PbO.sub.2 is formed. The .alpha.-PbO.sub.2 mainly serves as
a joint between the substrate/primary layer and the
.beta.-PbO.sub.2 coating layer described later. That is, since the
radius of metal ions of Pb.sup.4+ is greater by 0.1 to 0.2 .ANG.
than the radius of metal ions of titanium, tin, tantalum and
niobium in the primary layer or the metal substrate and, since all
are rutile type oxides and thus have identical crystal
configuration, misfitting between the oxides is great and can
possibly worsen the bondability. This problem can be moderated by
disposing .alpha.-PbO.sub.2 having a different crystal structure as
an intermediate layer therebetween. Accordingly, the
.alpha.-PbO.sub.2 can be thin so long as it can serve as the joint
and, since excessive thickness may possibly cause problems in the
corrosion resistance and electroconductivity, the appropriate
thickness is from about 20 to 500 .mu.m. There is no particular
restriction for the method of forming the .mu.-PbO.sub.2
intermediate layer, and a method of electrolytically forming the
layer from an aqueous alkaline solution containing Pb ions through
anodic oxidation is usually suitable. The intermediate layer
coating of a desired thickness can be obtained under typical
conditions of using an electrolyte in which lead monoxide (PbO) is
dissolved to saturation in an aqueous solution of 3 to 5N NaOH and
electrolysis is conducted with a current density of from 0.1 to 10
A/dm.sup.2, at a temperature of from 20.degree. to 60.degree. C.
under a voltage of from 1 to 2 V, for a time from 0.1 to 10 hours
using a substrate coated with the primary layer as the anode.
After coating the .alpha.-PbO.sub.2 intermediate layer in this way,
a coating layer comprising .beta.-PbO.sub.2 is formed on the
surface thereof. The .beta.-PbO.sub.2 layer has an extremely
satisfactory affinity with the .alpha.-PbO.sub.2 intermediate layer
and known methods of forming .beta.-PbO.sub.2 can be used to make
the coating layer. The .beta.-PbO.sub.2 layer can be formed with
ease by an electrolytic process using an acidic bath containing
lead ions, such as an aqueous 30-35% lead nitrate solution, as the
electrolyte and using a substrate coated with the primary layer and
the intermediate layer as the anode. Suitably the current density
is from 0.1 to 20 A/dm.sup.2 and the electrolysis time is from
about 0.1 to 10 hours. Although a slight amount of the
.alpha.-PbO.sub.2 layer intrudes into the coating layer comprising
.beta.-PbO.sub.2 using this method, the intrusion does not result
in any problems in view of the durability.
In the electrolytically formed .beta.-PbO.sub.2 layer, internal
strains of its own are necessarily produced in view of the crystal
structure, and these internal strains can desirably be releaved by
incorporating a corrosion resistant and electrochemically inactive
granular and/or fibrous material into the .beta.-PbO.sub.2 layer.
That is, by incorporating the granular and/or fibrous material into
the .beta.-PbO.sub.2 layer, continuous bonding of .beta.-PbO.sub.2
in the coating layer can be avoided to obtain an advantageous
effect of dispersing the internal strains formed in the
.beta.-PbO.sub.2 layer due to electrodeposition.
As the corrosion resistant and electrochemically inactive material
to be incorporated and dispersed in the .beta.-PbO.sub.2 coating
layer, any material can be used so long as the material is
corrosion resistant and exerts no effect on the electrochemical
activity of the .beta.-PbO.sub.2 layer. Metal oxides are generally
suitable since they are corrosion resistant and less reactive, and
oxides of metals of group IV and group V of the periodic table such
as Ti, Ta, Zr, Hf, Nb and V are particularly effective. Carbides,
nitrides or borides of these metals can also be used. Further,
fluoro resins can also be used suitably since they are excellent in
chemical resistance and show no reactivity. Those metals referred
to as valve metals exemplified above among the group IV and group V
elements can also be used in the form of a metal since they produce
passivated corrosion resistant oxide films at the surface by the
anodic oxidation and show no reactivity.
The content of these materials can properly be selected, and,
suitably, it is from about 0.01 to 10% by weight based on the total
amount of the coating layer. The granule or fiber diameter of these
materials is preferably less than 500 .mu.m.
There is no particular restriction for the method of forming such a
coating layer and it is suitable to employ an electrolytic forming
method combined with a so-called dispersed plating method in which
a .beta.-PbO.sub.2 layer is electrolytically formed while
dispersing the granular and/or fibrous material in the electrolyte.
Further, formation of the .beta.-PbO.sub.2 layer and the
introduction of the above-mentioned material can be conducted
separately in an appropriate order. That is, a coating layer
comprising a .beta.-PbO.sub.2 layer and a layer of the fibrous
and/or granular material alternately may be formed by repeating,
for several times, the procedures of forming a thin
.beta.-PbO.sub.2 layer electrolytically, coating the fibrous and/or
granular material thereover and then baking. Conventional methods
can be used for the electrolytic conditions and, usually,
electrolysis may be conducted in a lead nitrate bath while using
the intermediate-coated substrate as the anode, preferably, with
the current density from 0.1 to 10 A/dm.sup.2 and at a temperature
of from about 40.degree. to 80.degree. C.
In this way, a lead oxide-coated electrode having a
.beta.-PbO.sub.2 layer as the electrode active surface can be
obtained with ease.
This invention will now be described referring to examples but the
invention is in no way limited only thereto. Unless otherwise
specified, all percents, ratios, etc. are by weight.
EXAMPLE 1 AND COMPARATIVE EXAMPLES 1 TO 3
The surface of an expanded mesh made of pure titanium of 1.5 mm
plate thickness was blasted by using #70 stainless steel grits
(average grain size: 0.7 mm) and washed for 15 min. in a boiling
aqueous 25% hydrochloric acid solution. Then, using the titanium
expanded mesh as the substrate, a primary layer comprising platinum
and tantalum oxide in Pt/Ta=1/1 (metal molar ratio) composition was
disposed on the surface thereof to a thickness of 0.1 .mu.m. The
primary layer was formed by using platinum in the form of
chloroplatinic acid and tantalum in the form of tantalum
pentachloride dissolved in an aqueous 4% hydrochloric acid solution
used as the coating solution for the primary layer. and repeating 4
times the procedures of coating the solution by brushing onto the
expanded mesh of the substrate, drying at 40.degree. C. and then
heating in a muffle furnace at 500.degree. C. for 10 minutes.
Then, electrolysis was conducted using the thus formed primary
layer as the anode and a titanium plate as the cathode, in an
electrolyte comprising lead monoxide (PbO) dissolved to saturation
in an aqueous 3.5N sodium hydroxide solution at 40.degree. C., with
a current density of 1 A/dm.sup.2 for two hours thereby forming an
.alpha.-PbO.sub.2 coating layer as the intermediate layer. The
thickness of the intermediate layer was about 100 .mu.m.
Further, a lead dioxide layer composed of .mu.-PbO.sub.2 was formed
as the surface coating layer by the electrolytic method as
described below. That is, electric current was supplied using an
aqueous 30% lead nitrate solution as an electrolyte and using a
titanium plate as a cathode while stirring the solution with a
magnetic stirrer, at a temperature of from 65.degree. to 70.degree.
C. with a current density of 2 A/dm.sup.2 for 2 hours. Thus, an
electrode having a .beta.-PbO.sub.2 coating layer of about 200
.mu.m thickness was obtained.
As comparative Examples, specimens were prepared in the same manner
as described above except for deleting the platinum - tantalum
oxide primary layer (Comparative Example 1), deleting the
.alpha.-PbO.sub.2 intermediate layer (Comparative Example 2) and
using only the surface coating .beta.-PbO.sub.2 layer (Comparative
Example 3).
An accelerated electrolysis test was conducted on the specimens
using them as the anode in an aqueous 150 g/l sulfuric acid
solution at 60.degree. C. with a current density of 200
A/cm.sup.2.
The results are shown in Table 1.
TABLE 1 ______________________________________ Electrode Coatings
Inter- Primary mediate Coating Life time No. Layer Layer Layer (hr)
______________________________________ Example 1 Pt--Ta oxide
.alpha.-PbO.sub.2 .beta.-PbO.sub.2 more than 300 Comparative --
.alpha.-PbO.sub.2 .beta.-PbO.sub.2 5 Example 1 Comparative Pt--Ta
oxide -- .beta.-PbO.sub.2 67 Example 2 Comparative -- --
.beta.-PbO.sub.2 3 Example 3
______________________________________
As can be seen from the Table 1, for the specimens with no primary
layer (Comparative Examples 1 and 3), current conduction became
impossible within 3 to 5 hours from the beginning and the coating
peeled from the substrate. In the specimen in which no intermediate
layer was disposed but the surface coating layer was disposed
directly above the primary layer (Comparative Example 2), although
a certain life time was recognized, the coating layer peeled early
during electrolysis since the bondability was poor between the
.beta.-PbO.sub.2 (coating layer) and the primary layer. On the
other hand, the electrode according to the present invention
(Example 1) showed neither weight reduction nor peeling during
electrolysis for more than 300 hours and electrolysis could be
conducted stably for a long period of time.
EXAMPLE 2
A titanium substrate was prepared in the same manner as in Example
1. A coating composed of tantalum oxide and titanium oxide (metal
molar ratio 1/2) was first formed on the surface to a thickness of
about 0.1 .mu.m and then a coating composed of platinum and an
oxide mixture of tantalum oxide and titanium oxide (metal molar
ratio: 3/1/2) was formed to a thickness of about 0.1 .mu.m to form
a primary layer. The primary layer was formed by coating an aqueous
hydrochloric acid solution of tantalum pentachloride and titanium
tetrachloride, and an aqueous hydrochloric acid solution of
chloroplatinic acid, tantalum pentachloride and titanium
tetrachloride on the substrate, respectively, drying and then
heating at 550.degree. C. in air. The procedures of coating and
heating were repeated twice for each of the coatings. Next an
.alpha.-PbO.sub.2 layer was disposed in the same manner as in
Example 1 over the primary layer. The electrolysis was conducted
for one hour to form the .alpha.-PbO.sub.2 layer to a thickness of
about 100 .mu.m.
Then, a coating layer composed of .beta.-PbO.sub.2 was
electrolytically formed over the .alpha.-PbO.sub.2 layer.
Electrolysis was conducted using an aqueous 35% lead nitrate
solution as an electrolyte under stirring by passing nitrogen gas
through the solution with a current density of 2 A/dm.sup.2 for 2
hours to obtain a coating layer of about 500 .mu.m thickness. The
electrolysis temperature was 50.degree. C.
When conducting an accelerated electrolysis test in the same manner
as in Example 1 for the specimen electrode, it was found that there
was no change in voltage increase, etc. even for continuous
electrolysis for more than 100 hours and it could be used with
extreme stability.
EXAMPLE 3
A titanium substrate was prepared in the same manner as in Example
1. A primary layer composed of palladium oxide and tin oxide was
formed on the surface. The primary layer was formed by using a
coating solution prepared by dissolving palladium chloride in an
n-amyl alcohol solution on n-amyl alkoxy tin, coating the solution
on the substrate by brushing, drying at 150.degree. C. and then
baking at 500.degree. C. The procedures were repeated twice to form
a primary coating of 0.2 .mu.m thickness. Further, the product was
placed in a muffle furnace at 600.degree. C. and sintered to
stabilize for 2 hours. Then, an .alpha.-PbO.sub.2 layer of about
200 .mu.m thickness was formed in the same manner as in Example
1.
A lead oxide layer composed of .beta.-PbO.sub.2 was
electrolytically formed as the coating layer on the
.beta.-PbO.sub.2 layer. The electrolysis was conducted under the
same conditions as in Example 2 for 4 hours to obtain a
.beta.-PbO.sub.2 coating layer of about 1 mm thickness.
An accelerated electrolysis test was conducted for the specimen in
the same manner as in Example 1 in an aqueous 150 g/l H.sub.2
SO.sub.4 solution at 60.degree. C. As a result, after conducting
electrolysis with an electric current density of 100 A/dm.sup.2 for
more than 400 hours, there was neither substantial weight change
nor cracking in the coating layer. Only discoloration observed at
the surface.
EXAMPLE 4 AND COMPARATIVE EXAMPLES 4-6
The surface of an expanded mesh made of pure titanium of 1.5 mm
plate thickness was blasted by using #70 stainless steel grits
(average grain size: 0.7 mm) and washed for 15 min. in a boiling
aqueous 25% hydrochloric acid solution. Then, using the titanium
expanded mesh as the substrate, a primary layer comprising platinum
and tantalum oxide in Pt/Ta=1/1 (metal molar ratio) composition was
disposed on the surface thereof to a thickness of 0.1 .mu.m. The
primary layer was formed by using platinum in the form of
chloroplatinic acid and tantalum in the form of tantalum
pentachloride dissolved in an aqueous 4% hydrochloric acid solution
used as the coating solution for the primary layer, and repeating 4
times the procedures of coating the solution by brushing on the
expanded mesh of the substrate, drying at 40.degree. C. and then
heating in a muffle furnace at 570.degree. C. for 10 minutes.
Then, electrolysis was conducted using the thus formed primary
layer as the anode and a titanium plate as the cathode, in an
electrolyte comprising lead monoxide (PbO) dissolved to saturation
in an aqueous 3.5N sodium hydroxide solution at 40.degree. C., with
a current density of 1 A/dm.sup.2 for two hours thereby forming an
.alpha.-PbO.sub.2 coating layer as the intermediate layer. The
thickness of the intermediate layer was about 100 .mu.m.
Further, a lead dioxide layer composed of .beta.-PbO.sub.2
containing a fluoro resin was formed as the surface coating layer
under the conditions described below. An electrolyte was prepared
by adding 10 ml of a fluoro resin dispersion (trade name, "Teflon
30J" manufactured by Mitsui Du Pont Fluoro Chemical) per 1 liter of
an aqueous 30% solution of lead nitrate. Electric current was
supplied while using a titanium plate as a cathode under stirring
the solution by passing nitrogen gas therethrough at a temperature
of from 65.degree. to 70.degree. C. with a current density of 2
A/dm.sup.2 for 2 hours. Thus, a lead oxide layer containing fluoro
resin of about 300 .mu.m thickness was obtained.
As comparative electrodes, specimens were prepared in the same
manner as above except for deleting the platinum-tantalum oxide
primary layer (Comparative Example 4), deleting the
.alpha.-PbO.sub.2 intermediate layer (Comparative Example 5) and
using only the surface coating layer (Comparative Example 6).
An accelerated electrolysis test was conducted on the specimens
using them as the anode in an aqueous 150 g/l sulfuric acid
solution at 60.degree. C. with a current density of 100
A/cm.sup.2.
The results are shown in Table 2.
TABLE 2 ______________________________________ Electrode Coatings
Inter- Primary mediate Coating Life time No. Layer Layer Layer (hr)
______________________________________ Example 4 Pt--Ta
.alpha.-PbO.sub.2 Fluoro resin- more than oxide containing 500
.beta.-PbO.sub.2 Comparative -- .alpha.-PbO.sub.2 Fluoro resin- 64
Example 4 containing .beta.-PbO.sub.2 Comparative Pt--Ta -- Fluoro
resin- 145 Example 5 oxide containing .beta.-PbO.sub.2 Comparative
-- -- Fluoro resin- 30 Example 6 containing .beta.-PbO.sub.2
______________________________________
As can be seen from Table 2, for the specimens with no primary
layer (Comparative Examples 4 and 6), current condition became
impossible within a short period of time and the coating peeled
from the substrate.
In the specimen in which no intermediate layer was disposed but the
surface coating layer was disposed directly above the primary layer
(Comparative Example 5), although a certain life time was
recognized, cracking was formed soon during electrolysis. On the
other hand, the electrode according to the present invention
(Example 4) showed neither weight reduction nor peeling during
electrolysis for more than 500 hours and electrolysis could be
conducted stably for a long period of time.
EXAMPLE 5
A titanium substrate was prepared in the same manner as in Example
4. A coating composed on tantalum oxide was at first formed on the
surface to a thickness of about 0.1 .mu.m and then a coating
composed of an oxide mixture of palladium oxide and tantalum oxide
was formed to a thickness of about 0.1 .mu.m to form a primary
layer. The primary layer was formed by coating an aqueous
hydrochloric acid solution of tantalum pentachloride and an aqueous
hydrochloric acid solution of palladium chloride and tantalum
pentachloride on the substrate, respectively, drying and then
heating them 550.degree. C. for 10 min. in air. The procedures of
coating and heating were repeated three times for each of the
coatings. An .alpha.-PbO.sub.2 layer was disposed in the same
manner as in Example 4 over the primary layer. The electrolysis was
conducted for one hour to form the .alpha.-PbO.sub.2 layer to a
thickness of about 100 .mu.m.
Then, a coating layer composed of .beta.-PbO.sub.2 containing
niobium oxide dispersed therein was electrolytically formed on the
.alpha.-PbO.sub.2 layer. An aqueous 35% lead nitrate solution
containing 10 g of fine niobium oxide fully passing 345 mesh
dispersed per 1 liter of the solution was used as an electrolyte.
Electrolysis was conducted using the electrolyte under stirring by
using a magnetic stirrer with a current density of 4 A/dm.sup.2 for
2 hours to obtain a coating layer of about 1 mm thickness. The
electrolysis temperature was 40.degree. C.
When conducting an accelerated electrolysis test in the same manner
as in Example 4 for the specimen in 150 g/l sulfuric acid, it was
found that there was neither a voltage increase, nor peeling of the
coating layer at all even for electrolysis for more than 500 hours
with a current density of 100 A/dm.sup.2.
EXAMPLE 6
A titanium substrate was prepared in the same manner as in Example
4. A primary layer composed of platinum and tin oxide was formed on
the surface. The primary layer was formed by using a coating
solution prepared by dissolving chloroplatinic acid in n-amyl
alcohol solution of n-amyl alkoxy tin, coating the solution on the
substrate by brushing, drying at 150.degree. C. and then baking at
500.degree. C. The procedures were repeated twice to form a primary
coating of 0.2 .mu.m thickness. Then, an .alpha.-PbO.sub.2 layer of
about 200 .mu.m thickness was formed in the same manner as in
Example 4.
A lead oxide layer composed of .beta.-PbO.sub.2 containing titanium
dispersed therein was electrolytically formed as the coating layer
on the .alpha.-PbO.sub.2 layer. Titanium sponge pulverized in
ethanol into products fully passing 275 mesh was used as titanium.
The electrolysis was conducted under the same conditions as in
Example 5 for 4 hours to obtain lead oxide-coated electrode having
a .beta.-PbO.sub.2 coating layer of about 2 mm thickness.
An accelerated electrolysis test was conducted for the specimen in
the same manner as in Example 4 using an aqueous 150 g/l H.sub.2
SO.sub.4 solution. As a result, after conducting electrolysis for
700 hours, there was neither substantial weight change nor cracking
in the coating layer. Only discoloration was observed at the
surface.
In the present invention, since corrosion resistant metal is used
for the electrode substrate and a primary layer comprising platinum
and/or palladium oxide, an intermediate layer comprising
.alpha.-PbO.sub.2 and a coating layer comprising .beta.-PbO.sub.2
are successively coated thereover, the layers are firmly adhered to
the substrate thereby enabling the obtainment of a lead
oxide-coated electrode with no strains due to electrodeposition,
being strong and having high durability. In addition, passivation
and the resistance increase of the electrode can be prevented, and
the electrode according to the present invention can be used stably
for a long period of time even during electrolysis at high current
density, which is extremely useful as an electrode for various
electrolysis or electrolytic treatments requiring high corrosion
resistance and high oxygen overvoltage.
While the invention has been described in detail and with reference
to specific embodiments thereof, it will be apparent to one skilled
in the art that various changes and modifications can be made
therein without departing from the spirit and scope thereof.
* * * * *